Its an '04 and you want to replace the timing belt? Well first, its a chain, and its about $300 bux from Toyota, and not easy to replace. Unless you've at 125k miles, its fine.
I'd check plugs first, and your tension on the serpentine belt, if you're having problems the tensioner probably needs replaced (only if its squeeking). Next I'd have your timing checked first. I can get you the actual Toyota manuals for any work you'd need to do.
Chain
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Your Toyota Previa has a chain and not a belt the should last the life of the engine and does not normally need replacing.
Toyota recomends replacing the timing belt on this vehicle at 90,000 miles, I belive that is about 145,000 kilometers
The 2.4 liter four cylinder in a 2004 Toyota Camry has a timing CHAIN
Your 2004 Toyota RAV4 has a timing chain that is designed to last the life of the engine.
http://www.toyota.com/help/faq-service.html 2004 Toyota Mtrix has timing chain
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In a 2004 Toyota 4 Runner : The 4.7 liter V8 engine has a timing BELT
Your 2004 Toyota RAV4 has a timing chain that is designed to last the life of the engine.